Conventionally, as ink used for writing and recording on a material to be recorded, such as paper, or for ink-jet printing, various dyes or pigments which are dissolved or dispersed in liquid medium or binder have been used.
An ink obtained by dispersing organic pigment in a solvent has an advantage in excellent fastness such as light fastness. However, the pigment in this ink deposits or agglomerates with time and special treatment in dispersion process is therefore necessary.
Oil ink obtained by dissolving a dye in an organic solvent (e.g. hydrocarbons) is more profitable as it does not require such a troublesome dispersion process. In this ink, however it has recently been desired that the organic solvent is changed to alcohols or glycols which are low in toxicity to the human body and therefore the dye must have excellent solubility with alcohol.
As the dyes which satisfy such demand as much as possible, there are known various alcohol soluble dyes, for instance, a salt of basic dye and acid dye, a basic dye treated by fatty acid etc., a metal complex dye having a water soluble group such as a sulfonic group or an acid dye treated by an organic amine. However, the oil ink using the above listed alcohol soluble dyes (for instance, amine salt dye of acid yellow 23, 42; acid red 73, 180; direct blue 87; acid black 52 or salt-forming dye using basic dye such as auramine, rhodamine, victoria blue, methyl violet, etc.) are not satisfactory in regard to stability of ink and fastness of written image (light fastness, water resistance etc.), resulting in that the dye deposits with time or the ink increases viscosity when environmental conditions such as temperature and humidity change.